In oil wells, natural gas wells etc. (hereinafter collectively referred to as “oil wells”), oil country tubular goods such as casing and tubing are used to produce underground resources. Oil country tubular goods are series of steel pipes which are connected together, and these steel pipes are connected by threaded joints.
Such threaded joints for steel pipe are generally categorized as coupling-type and integral-type. A coupling-type joint connects a pair of pipes, where one of the pipes is a steel pipe and the other pipe is a coupling. In this case, a male thread is provided on the outer periphery of both ends of the steel pipe, while a female thread is provided on the inner periphery of both ends of the coupling. Then, the male thread of the steel pipe is screwed into the female thread of the coupling such that they are fastened together and connected. An integral-type joint connects a pair of pipes that are both steel pipes, and does not use a separate coupling. In this case, a male thread is provided on the outer periphery of one end of a steel pipe, while a female thread is provided on the inner periphery of the other end thereof. Then, the male thread of one steel pipe is screwed into the female thread of another steel pipe such that they are fastened together and connected.
Generally, an end portion of a pipe that includes a male thread and serves as a joint portion is referred to as pin, since it includes an element to be inserted into the female thread. On the other hand, an end portion of a pipe that includes a female thread and serves as a joint portion is referred to as box, since it includes an element that receives the male thread. Since the pin and box are end portions of a pipe, they are tubular in shape.
FIG. 1 is a vertical cross-sectional view of an example of a typical conventional threaded joint for steel pipe. The threaded joint shown in FIG. 1 is a coupling-type threaded joint and includes a pin 110 and a box 120.
The pin 110 includes, beginning with its tip toward its base, a shoulder surface 111, a sealing surface 113 and a male thread 114. The box 120 includes, beginning with its base toward its tip, a shoulder surface 121, a sealing surface 123 and a female thread 124. The shoulder surface 121, sealing surface 123 and female thread 124 of the box 120 are provided so as to correspond to the shoulder surface 111, sealing surface 113 and male thread 114 of the pin 110. The male thread 114 of the pin 110 and the female thread 124 of the box 120 engage each other, and the threaded portions having these threads are trapezoidal threads constituted by taper threads.
The male thread 114 and female thread 124 allow screw-in to each other, and are in tight contact by fitting together upon fastening, thereby achieving an interference fit. As the pin 110 is screwed in, the sealing surfaces 113 and 123 come into contact, are in close contact by fitting together upon fastening, thereby achieving an interference fit. Thus, the sealing surfaces 113 and 123 form a seal portion by metal-to-metal contact. As the pin 110 is screwed in, the shoulder surfaces 111 and 121 come into contact and are pressed against each other, and thus serve as stoppers for limiting the screw-in of the pin 110. Upon fastening, the shoulder surfaces 111 and 121 serve to apply a so-called thread-tightening axial force to the load flank of the male thread 114 of the pin 110.
In a threaded joint with this construction, in addition to the close contact by fitting together between the male thread 114 and female thread 124, the close contact by fitting together between the sealing surfaces 113 and 123 ensure sealing performance.
In recent years, land and offshore wells have become deeper and deeper, which means that oil-well environments have become harsher and harsher with higher temperatures and higher pressures as well as higher corrosion levels. To address such harsh environments, the oil country tubular goods used are often heavy wall steel pipes. A threaded joint for connecting such steel pipes is required to have good sealing performance against an internal pressure and an external pressure.
Conventional techniques for improving the sealing performance of a threaded joint include the following.
FIG. 2 is a vertical cross-sectional view of an example of a conventional threaded joint for steel pipe designed to improve the sealing performance. The threaded joint shown in FIG. 2 includes two seal portions with metal-to-metal contact. Further, shoulder surfaces are provided near the middle of the threaded joint as determined along the pipe-axis direction (see, for example, U.S. Pat. No. 4,662,659).
More specifically, as shown in FIG. 2, the pin 210 includes, beginning with its tip toward its base, a first sealing surface 213, a first male thread 214, a shoulder surface 211, a second sealing surface 216 and a second male thread 217. The box 220 includes, beginning with its base toward its tip, a first sealing surface 223, a first female thread 224, a shoulder surface 221, a second sealing surface 226 and a second female thread 227. The first threaded portion composed of the first male and female threads 214 and 224 and the second threaded portion composed of the second male and female threads 217 and 227 are trapezoidal threads constituted by taper threads.
The taper surface defined by the first threaded portion is closer to the pipe axis CL than the taper surface defined by the second threaded portion because shoulder surfaces 211 and 221 are provided between the first and second threaded portions.
The first male and female threads 214 and 224 allow screw-in to each other, and are in close contact by fitting together upon fastening, thereby achieving an interference fit. The second male and female threads 217 and 227 also achieve an interference fit. As the pin 210 is screwed in, the first sealing surfaces 213 and 223 come into contact and the second sealing surfaces 216 and 226 come into contact, and the first sealing surfaces 213 and 223 and the second sealing surfaces 216 and 226 are in close contact by fitting together each other upon fastening, thereby achieving an interference fit. As the pin 210 is screwed in, the shoulder surfaces 211 and 221 come into contact and are pressed against each other.
In a threaded joint with this construction, the close contact by interference fit between the first sealing surfaces 213 and 223 ensures sealing performance mainly against an internal pressure. Further, the close contact by interference fit between the second sealing surfaces 216 and 226 ensures sealing performance mainly against an external pressure.